The present invention relates to electrochromic devices and more particularly relates to solid-state, inorganic thin film electrochromic devices.
Electrochromic materials and devices have been developed as an alternative to passive coating materials for light and heat management in building and vehicle windows. In contrast to passive coating materials, electrochromic devices employ materials capable of reversibly altering their optical properties following electrochemical oxidation and reduction in response to an applied potential. The optical modulation is the result of the simultaneous insertion and extraction of electrons and charge compensating ions in the electrochemical material lattice.
In general, electrochromic devices have a composite structure through which the transmittance of light can be modulated. FIG. 1 illustrates a typical five layer solid-state electrochromic device in cross-section having the five following superimposed layers: an electrochromic electrode layer (“EC”) 14 which produces a change in absorption or reflection upon oxidation or reduction; an ion conductor layer (“IC”) 13 which functionally replaces an electrolyte, allowing the passage of ions while blocking electronic current; a counter electrode layer (“CE”) 12 which serves as a storage layer for ions when the device is in the bleached or clear state; and two conductive layers (“CL”) 11 and 15 which serve to apply an electrical potential to the electrochromic device. Each of the aforementioned layers are typically applied sequentially on a substrate 16. Such devices typically suffer from electronic leakage and electronic breakdown.
In order for a solid state electrochromic device to function correctly, it is necessary to incorporate an ion conductor layer which effectively blocks electronic current while admitting the passage of ions (typically protons (H+) or lithium ions (Li+)) at a reasonable rate. However, any electronic current that leaks or passes through the ion conductor layer serves to short out the required voltage and inhibits the flow of ions. As such, electronic leakage current leads to compromises in device performance including a lowered dynamic range, non-uniform coloration, decreased ionic conductance, slower switching rates, and increased power consumption. Merely increasing the thickness of the ion conductor layer may result in a reduction of leakage current, but at the expense of degraded optical properties, increased layer deposition time and cost, and reduced switching rates. Accordingly, it is desirable to reduce the amount of electronic leakage through an electrochromic device without resorting to a thick ion conductor layer so as to avoid these compromises in performance.
Moreover, solid state electrochromic devices are also subject to “electronic breakdown.” FIG. 2 shows the equilibrium electrical and optical characteristics of an electrochromic device for applied voltages in the ‘forward’ or coloring polarity, up until the point where substantial electronic leakage current begins to flow. These characteristics are plotted as a function of the internal voltage, which is the voltage applied to the device minus the voltage induced in the series resistance as a result of the current flowing. Initially, there is a slow increase in the current density below a particular ‘threshold voltage,’ accompanied by an increase in optical density. Above the threshold voltage, however, the electrochromic device breaks down electronically and any additional current flowing is primarily electronic leakage. Moreover, reaching the threshold voltage prevents any further increases in voltage across the electrochromic device. Since the coloration of a device is related to the voltage that can be developed across the electrochromic device, the threshold voltage will determine the maximum obtainable optical density for a given device structure, i.e. a particular layer thickness, and, hence its dynamic range. As such, it is desirable to increase the voltage that can be developed across the ion conductor layer of the electrochromic device prior to it breaking down so as to allow electrochromic devices to color more deeply and to reduce power consumption.
Takahashi (U.S. Pat. No. 4,293,194) discloses a device having decreased electronic leakage. Takahashi teaches a solid electrochromic device incorporating an electron blocking material comprised of a layer of an N-type semiconductor adjacent to a layer of a P-type semiconductor. However, Takahashi does not teach a multi-layered thin film ion conductor layer capable of reducing electronic leakage while increasing the voltage that may be developed across the ion conductor layer.